Along with progress made in recent years in reducing the size of semiconductors, a new type semiconductor possessing a three-dimensional structure such as the Fin Field Emission Transistor (FinFET) has started to appear. Along with these new type semiconductors, the critical dimension measurement devices and scanning devices used for semiconductor production and research require the capability to observe in fine detail three-dimensional shapes such as the bottom surface of concavities and device sidewalls whose angles are nearly perpendicular and therefore difficult to view during observation from directly above. Scanning electron microscopes (SEM) that utilize an electron beam are used for the purpose of length measurement and semiconductor inspection where high resolution capability is needed. Critical dimension scanning electron microscopes (CD-SEM) for example, possess a high resolution of approximately 2 nm at an acceleration voltage of 1 kilovolt or lower. Observing the above described three-dimensional shapes in this type of devices, requires tilted illumination observation that views from an oblique position rather than viewing from directly above.
The tilted illumination observation method is considered a technique where: (1) the specimen stage is set to be inclined, (2) the SEM column is set to be inclined, (3) a separate tilted illumination observation column is installed, and (4) a beam is irradiated at an oblique angle, etc. Among these steps, (1) through (3) require a special mechanical mechanism. In the case of (4), a primitive tilted illumination causes the blurring of the beam, so there is a loss of resolution. Whereupon methods to correct this blurring included (4a) installing a compensator device (See JP-A-2006-54074), and (4b) making adjustments of the astigmatism, focus and aperture position, so as to cancel out aberrations due to the beam inclination (See Japanese Patent No. 3968334).
A method has been proposed for making precise measurements of the beam tilt angle by using a specimen whose structure is known (See JP-A-2005-183369). The beam tilt angle must be accurately known in order to enhance the accuracy of the three-dimensional reconstruction assembled from multiple tilted beam images.
Though not specifically intended for tilted illumination observation, a technique to measure aberrations of the lens by using the beam inclination was developed for the transmission electron microscope (TEM) (Ultramicroscopy Vol. 3, 1978, pp. 49-60).
Moreover, a technology that adjusted deviations in aberration correction conditions was disclosed in JP-A-2007-173132. When image shifting function is used in Cs corrected scanning transmission electron microscopes (STEM), there is a possibility that the aberration correction condition is changed. By extracting the electron beam spot shapes from the tilt scanning transmission images of the specimen, using the deconvolution method and arranging these spot shapes in a tableau shape according to the tilt azimuth, it is possible to detect the deviation in aberration conditions from the symmetry of the tableau. Then correcting the deviation in aberration compensation conditions by using a two-stage deflector device to adjust the deflection center point. This technology for forming the tilted electron beam is at most a technique for adjusting the Cs corrected STEM, and intended if the adjustment goes well, to ultimately obtain a high resolution image with a vertical incident beam. This technology requires multiple tilt azimuths, and moreover, it is necessary to evaluate the symmetry of the tableau.
However, the technique described above in (1) through (3) need to incline the specimen stage or the SEM column, so the distance between the specimen and the objective lens cannot be narrowed in order to secure the space. Moreover, resolution improvement technique using a retarding electrical field cannot be utilized, because the specimen and the objective lens are not placed in parallel during tilt observation. As a result, resolution is inferior in the tilt observation compared with the top down observation. Further, problems are that it costs to add the hardware of the specimen tilt mechanism, it takes time to the tilt operation; therefore, the observation throughput deteriorates when frequently performing tilted illumination observation.
The technology in (4a) an aberration corrector removes aberrations increasing via the tilt so that high resolution during tilting it theoretically possible but has the problem that current compensator devices are expensive and the adjustment technique is complex. The technology in (4b) is theoretically incapable of restoring the resolution even if using methods such as shifting the aperture or electro-optically adjusting the axis so as to decrease the blurring of the scan spot caused by the tilted beam irradiation. A particular problem is that the resolution deteriorates drastically when the tilt angle reaches several degrees, compared to observation from right above. Also, techniques such as switching frequently between tilted illumination observation and observation from right above to compare the images are difficult for adjusting many parts of column and have difficulty in reproducibility.
Moreover, even if the beam tilt angle can be measured with some accuracy, the resolution of the tilted image is poor, so it is inevitable to improve the solution to rebuild the three-dimensional image.
In order to resolve the above problems of the conventional art, the present invention has the object of providing an easily adjustable tilted illumination observation method possessing high speed, good reproducibility and a low cost, as well as a device using that method for restoring the resolution by image processing after blurring of the image during tilted illumination observation, to a resolution equivalent to observation from directly above.